Oil casing is usually under the complicated internal and external stress environment. The study and analysis on the influence of these factors on casing collapse resistance ability are very important, which can provide theoretical basis for casing strength design. Based on the stress characteristics of casing under external non-uniform load and internal pressure and the theory of material mechanics, a calculation model of hoop stress of ideal casing at unit length is established, the distribution regularity of hoop stress of internal and external casing surfaces is revealed. The calculation shows that the yield failure point is at the internal surface in circumferential direction of 90 angle with maximum horizontal crustal stress. A new formula of external collapse resistance ability under non-uniform external collapse load and internal pressure during casing buckling is derived from the failure point by Von Mises yield criterion. Calculation examples show that non-uniformity coefficient of external load has the greatest impact on external collapse resistance ability among all the influence factors on collapse resistance ability of casing. The larger the non-uniformity coefficient of external load is, the faster the collapse resistance ability of casing declines; under less external load non-uniformity, the external load bearing capacity of casing can be improved by increasing the internal pressure and decreasing casing diameter-thickness ratio.